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Abstract

Both self-cleanability and antireflectivity were achieved on quartz surfaces by forming
heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane self-assembled monolayer after
fabrication of nanostructures with a mask-free method. By exposing polymethylmethacrylate
spin-coated quartz plates to O2 reactive ion etching (RIE) and CF4 RIE successively, three well-defined types of nanopillar arrays were generated: A2,
A8, and A11 patterns with average pillar widths of 33 ± 4 nm, 55 ± 5 nm, and 73 ± 14 nm,
respectively, were formed. All the fabrication processes including the final cleaning
can be finished within 4 h. All nanostructured quartz surfaces exhibited contact angles
higher than 155° with minimal water droplet adhesiveness and enhanced transparency
(due to antireflectivity) over a broad spectral range from 350 to 900 nm. Furthermore,
A2 pattern showed an enhanced antireflective effect that extends to the deep-UV range
near 190 nm, which is a drawback region in conventional thin-film-coating approaches
as a result of thermal damage. Because, by changing the conditions of successive RIE,
the geometrical configurations of nanostructure arrays can be easily modified to meet
specific needs, the newly developed fabrication method is expected to be applied in
various optic and opto-electrical areas.

PACS codes: 06.60.Ei; 81.65.Cf; 81.40.Vw.

Keywords:

Background

Numerous studies of surface nanostructures have been conducted to investigate enhancement
of the properties of bulk materials to improve their selectivity, applicability, and
effectiveness. During the past decades, the technological basis for nanofabrication
has been developed by vigorous efforts to develop next-generation lithography for
highly resolved patterns up to the industrial level of semiconductor production. Nanofabrication
techniques using transparent materials such as quartz comprise one of the most attractive
approaches to optical and opto-electrical studies as well as to highly sensitive biosensor
fields since quartz is commonly employed in these fields [1-4].

Various methods are used to achieve antireflective property in quartz or glass to
increase light transmission. Single- or multilayered thin film coatings, porous coatings,
and fabrication of sub-wavelength nanostructures on surfaces using conventional lithography
have been the focus of many studies [5-7]. However, the aforementioned conventional methods have some drawbacks. For example,
it is difficult to maintain long-term stability of multilayer polymeric coatings because
multilayers are unstable in humid environments and temperature changes, and most polymeric
materials have strong absorption in the UV region [8]. Moreover, single- or multilayer fabrication methods are only effective for a narrow
spectral range. Lithography techniques have the shortcomings of being time consuming,
expensive, and restricted to small areas.

In comparison with thin-film-coated surfaces, directly patterned surfaces usually
guarantee good mechanical stability because they are free from adhesion problems and
tensile stress. Recently, a few techniques for the direct fabrication of sub-wavelength
nanostructures on quartz or glass surfaces have been attempted by several groups.
Lohmüller et al. and Christopher et al. used block copolymer micelle lithography with
reactive ion etching (RIE) and reported that array pattern of a quartz nanostructure
showed excellent antireflectivity and anti-fogging in UV and deep-UV region [1,9]. Li et al. applied nanosphere lithography using PS microspheres 210 nm in diameter
and reported broad spectrum antireflectivity from 300 to 800 nm and anti-fogging [10]. If a simpler and faster technique is available to obtain appropriate nanostructures,
it is highly desirable. Hein et al. reported an innovatively simple and fast method
of nanostructure fabrication on glass surfaces by performing RIE after deposition
of an approximately 10-nm-thin lithographically unstructured metallic layer onto the
surface [11]. We also reported a simple and fast mask-free approach to fabricate nanostructures,
which uses two-step RIE (O2 and CF4 RIE) of polymer-coated quartz in our previous studies [3,4].

Superhydrophobic behavior of a surface can also be achieved by introducing micro-
and/or nanostructures at the surface. A superhydrophobic surface is usually defined
as having a contact angle greater than 150°. The self-cleaning effect refers to cases
in which contaminant particles adhered to a surface are easily washed off with rolling
of water droplets. To have a self-cleaning effect, the surface should possess minimized
adhesion properties as well as superhydrophobicity [12-14].

The mimetic fabrication of a superhydrophobic surface was primarily inspired by the
self-cleanable leaves of the lotus, which have an array of protrusions on the surface
[15,16]. The fabrication of self-cleanable surfaces has received a great deal of attention
in various novel applications, such as for easy removal of undesirable contaminants
from the surface of semiconductors and solar cells, prevention of water corrosion
on the exterior skins of automobiles and building units, biomaterials used in clinical
therapies that require minimal contamination, no-mass-loss transport of water droplet
systems, and microarrays that require specific wetting properties of the substrates
for precise spotting [17]. Especially in dry condition, which is common in optical or opto-electric applications,
the superhydrophobic surface is expected to decrease the adhesion of dusts because
the surface energy in surface-air interface decreases as the surface becomes superhydrophobic.

We previously reported a systematic approach to obtain a superhydrophobic surface
with tunable adhesiveness and suggested a useful nanofabrication strategy for achieving
self-cleanability that involved fabrication of pillar arrays without dead-end nanopores
covered with low-surface-energy materials [12,14]. In this study, we demonstrate both the remarkable broad spectrum antireflectivity
including deep-UV region and self-cleanability in nanostructured quartz surfaces using
our mask-free fabrication method.

Methods

The nanopillar arrays with various pillar diameters and inter-pillar distances were
fabricated on quartz plates by a mask-free approach. A quartz wafer plate (Buysemi,
Suwon, Gyeonggi-do, Korea) was cleaned with piranha solution and then rinsed thoroughly
with deionized (DI) water. After heating at 100°C for about 5 min, 950 PMMA A2, 495
PMMA A8, and 950 PMMA A11 (MicroChem Corp., Newton, MA, USA) were each spin-coated
onto the surface at 4,000 rpm for 25 s. The expected thicknesses of the PMMA layers
reported in the technical support information of the MicroChem products are 50, 500,
and 800 nm for A2, A8, and A11, respectively.

Post-baking was conducted for 30 min at 170°C. The PMMA-coated quartz plates were
then subjected to reactive ion etching in O2 plasma for 1 min and CF4 plasma for 10 min at 250 W of RF power, 40 mTorr, and 40 sccm using a custom-made
RIE system. To remove the organic remnants from the quartz pattern, each pattern was
sintered at 1,000°C in a furnace for 1 h, cleaned with piranha solution, rinsed with
DI water, and dried. The surface of each pattern was covered with a self-assembled
heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane (HDFS; Gelest Inc., Morrisville,
PA, USA) monolayer using a 3-mM solution of HDFS in n-hexane to reduce the surface
energy. All the fabrication processes including the final cleaning and self-assembled
monolayer formation could be finished within 4 h.

The morphological images of the nanopillars were obtained by field emission scanning
electron microscopy (FESEM; Jeol JSM6710F, Jeol Ltd., Tokyo, Japan). The surface-wetting
properties were evaluated by an Easydrop goniometer (KRÜSS, Hamburg, Germany), and
the dynamic angle was evaluated using a DSA 100 goniometer (KRÜSS). The advancing
and receding contact angles were obtained from more than three points in each specimen
by the sessile drop technique. To investigate optical performance, we measured the
transmission and reflection properties of the nanostructured quartz plates using a
UV-visible spectrometer (Optizen 3220UV, Mecasys Co., Ltd., Daejeon, Korea).

Results and discussion

Figure 1 shows oblique FESEM images of the fabricated nanopillar arrays obtained for each
PMMA resist using the mask-free method. The widths of the quartz nanopillars with
A2, A8, and A11 PMMA resists were estimated to be 33 ± 4, 55 ± 5, and 73 ± 14 nm,
respectively, from the FESEM images. The heights of the pillars were 95 ± 10, 200 ± 15,
and 265 ± 15 nm, respectively. The average pillar diameters, pillar heights, and inter-pillar
distances were uniform all over the area (2.5 × 2.5 cm2) in each sample. These results demonstrate that it is feasible to systematically
control the dimensional features of the pillar pattern. Specifically, larger and higher
pillars can be formed by controlling the thickness of the PMMA resist. The mechanism
of the nanostructure formation was previously reported [4]. Explaining briefly, pillar-like nanostructures can be fabricated by the O2 and CF4 two-step RIE process because, by controlling the RIE conditions appropriately, CF4-resistant CxFy polymeric mask is automatically and selectively deposited during the CF4 RIE process on the top of the dot-like nanostructures of the PMMA resist, which are
formed during the preceding O2 RIE process.

Table 1 shows the static, advancing, and receding water contact angles of the HDFS-modified
nanopillar quartz patterns. Photographs of the static water droplets on each patterned
surface are also shown in Figure 2. The static contact angles were measured at more than three points in each specimen,
and the average values were acquired. A quartz surface is known to be hydrophilic
due to the existence of hydroxyl groups on the surface, which is the reason why a
quartz surface is easily contaminated by dusts, because nature wants to decrease interfacial
energy between the hydrophilic quartz and the most hydrophobic air. In order to make
the quartz surface hydrophobic, so as to decrease the interfacial energy, a covalently
immobilized monolayer of HDFS molecules was formed on its surface, and the hydrophobicity
could be further increased to superhydrophobicity by the nanostructures. All of the
HDFS-treated nanopatterns had dynamic contact angles (Table 1) in the superhydrophobic range (greater than 150°) with low hysteresis of about 10°,
which demonstrates the self-cleaning effect (Additional file 1: Video S1). In contrast, the HDFS-modified surface of a plain quartz has an advancing
angle of about 120° with large hysteresis (40.0°).

In nature, some insects have sub-wavelength scale structure patterns with nipple-like
or tapered profiles on the cornea that exhibit a gradient in the refractive index
between the air and tissue interface. These characteristics play an important role
in increasing light transmission. Theoretically, for a thin-film coating, overall
reflectance can be a function of antireflection (AR) layer thickness d and the wavelength λ. For a graded-index transition, substantial antireflection can be obtained when the
ratio d/λ is about 0.4 or higher [1,10]. To enhance the transmission of light and suppress the reflection, the structural
size has to be in the sub-wavelength range. In the spectral region from UV to visible
light, the structural dimension has to be smaller than 200 nm [1,9].

Figure 3 shows the transmission properties of the structured quartz manufactured using A2,
A8, and A11 PMMA resists. Transmission data from unstructured quartz samples were
used as a reference. All structured quartz prepared using A2, A8, and A11 showed improved
transmission of about 2% to 3% over unstructured quartz in a broad spectral range
from the UV to the infrared (IR) region (350 to 900 nm). The structured quartz manufactured
using an A2 resist demonstrated transmission superior to the unstructured quartz even
in the deep-UV range from 190 to 300 nm (Figure 4). This deep-UV range is usually not covered by the conventional polymer AR-coating
method. The antireflective property of the structured quartz using A2 varies from
approximately 4.2% improvement at around 193 nm to 2.3% at around 340 nm, as indicated
by the black arrow. The transmission of the structured surface using A8 and A11 is
lower than that of the unstructured quartz below 300 nm, and this is partly the result
of light scattering introduced during the fabrication process. In the region from
the visible (350 nm) to IR (900 nm) range, the nanostructured quartz prepared using
A8 exhibited a stable and uniform antireflective effect and a better optical performance
above the 700-nm region than that obtained using the quartz prepared using A2. These
experimental results are in good agreement with the aforementioned theories related
to the height of nanopillars.

Figure 3.Transmission properties of structured quartz as determined by UV-visible spectrometry. Prepared using PMMA A2, PMMA A8, and PMMA A11 with unstructured quartz as a reference.

Conclusions

The mask-free method presented here may have several advantages. First, long-term
stability is expected because of the superhydrophobic self-cleaning effect. Second,
although the area can be restricted by the stage size of a RIE device, a large area
can be achieved because our method requires no masks. Figure 5 shows a large patterned area of 3.0 × 3.0 cm2. Third, our technology can be employed for industrial optical devices, optical components,
and interior and exterior materials requiring both self-cleanable and antireflective
properties. In addition, the manufacturing cost is minimal since the fabrication process
is simple and fast, and requires no mask.

Figure 5.Water droplets on a large surface of the superhydrophobic quartz.

In the near future, after proper optimization, we plan to apply our technique to other
optical components, such as lenses and optical filters, to demonstrate the wide applicability
of our technology. Furthermore, we will determine whether the antireflective spectral
regions can be controlled using different PMMA resists by changing the parameters
of the surface structures such as height, width, and pitch.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

JSK prepared the samples, evaluated surface characteristics, and drafted the manuscript.
HWJ evaluated the optical properties, analyzed the data, and drafted the manuscript.
WL participated in the design of the study and drafted the manuscript. BGP evaluated
the surface characteristics of the samples. BMK and KBL conceived of the study together
and participated in its design and coordination. All authors read and approved the
final manuscript.

Acknowledgements

This study was supported in part by the National Research Foundation of Korea (NRF)
grant funded by the Korean government (MEST) (no. 210–0014693), the Seoul R&BD Program
(10920), the Korea Science and Engineering Foundation through the Pioneer Converging
Technology Program (no. M1071160001-08 M1116-00110), and by a grant of the Korea Healthcare
Technology R&D Project, Ministry of Health, Welfare & Family Affair, Republic of Korea
(A084204). This study was also supported by a grant from the Korean Health Technology
R&D Project, Ministry for Health, Welfare & Family Affairs (A102024) and a grant (20110027241)
from the Basic Research Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and Technology, Republic of Korea.